A TDD-TDMA scheme has been known as an air interface employed in a radio communication system which forms a service area divided into multiple cells (sectors) and includes multiple base station devices. In the radio communication system employing the TDD-TDMA scheme (for example, Personal Handyphone System), an influence of fading in a communication path hinders increase of the communication capacity.
Multipath fading, in particular, is caused by the presence of several delay waves. In the communication path having multi-paths, characteristics of a propagation path can be improved by equalizing the delay waves. In the communication system employing TDD, the same frequency is used for downlink and uplink communication paths. For this reason, for example, when a mobile station (terminal) equalizes the delay waves on the uplink communication channel, a delay spread estimated based on a signal received on the downlink communication channel can be applied to a delay spread for the uplink communication channel without any changes.
Patent Document 1 discloses an asymmetric radio communication method in which the communication speed of the downlink is higher than that of the uplink. Specifically, the asymmetric radio communication method employs orthogonal frequency-division multiplexing (OFDM) for the downlink. An IDFT input signal used in such an OFDM-compliant modulator is provided with such characteristics that are the uplink characteristics of frequency characteristics of the propagation path which are estimated from a signal on the uplink (uplink communication channel).
In addition, Patent Document 1 also discloses the following application of such a method. Specifically, after a downlink modulated wave modulated according to OFDM is demodulated, the demodulated signal is subjected to a Fourier transform, to thereby estimate the frequency characteristics of the propagation path.
By contrast, when digital modulation, such as Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), and Quadrature Amplitude Modulation (QAM), is performed on the uplink, the uplink communication quality can be improved by compensating the characteristics of the uplink propagation path. This compensation is accomplished by superimposing time characteristics (delay profile) on a baseband filter of a digital modulator. The time characteristics are created by performing an inverse Fourier transform on the estimated frequency characteristics.
Meanwhile, the following technique is introduced into the radio communication system employing TDD. Specifically, the directivity of an antenna is controlled on the base station side by using a technique such as an adaptive array antenna. In general, when the radio communication system using TDD employs the adaptive array technique, the base station sets the directivity of the antenna to be optimum according to the characteristics of the uplink propagation path, and then transmits a downlink signal by using the directivity thus set.
To be more specific, the base station optimizes the directivity of the adaptive array antenna by using a known signal pattern (a preamble and an uplink pilot) included in an uplink TDD frame repeatedly transmitted in predetermined cycles. The base station transmits and receives user data by using a pattern (weight) of the optimized directivity.
Patent Document 1: Japanese Patent Application Publication No. 2002-141888